Agriculture Reference
In-Depth Information
TABLE 1.20
Photosynthetic Rate per Unit Leaf Area in Various Crop Plants
Photosynthetic Rate
(mg CO
2
dm
−
2
h
−
1
)
Crop Plant
Alfalfa
50-55
Bean (common)
30-40
Maize
60-80
Pea (field)
25-30
Potato
25-35
Rice (lowland)
40-50
Soybean
30-35
Sugarbeet
30-35
Sunflower
40-42
Wheat
28-39
Source:
Adapted from Mahon, J. 1983.
Can. J. Plant Sci
. 63:11-21; Tanaka, A. and
M. Osaki. 1983.
Soil Sci. Plant Nutr
. 29:147-158. With permission.
larger fractions of N (about 25%) become components of the enzyme Rubisco (ribulose-bisphosphate
carboxylase). The rates of photosynthesis are therefore closely correlated with Rubisco activity in
leaves (Evans, 1983). The net photosynthesis of individual rice leaves reaches maximum values of
about 40-60 kilolux (800-1200 mol m
−2
s
−1
) near half full sunlight (Yoshida, 1981). However, the
photosynthesis of well-developed canopies increase with increasing light intensity of up to full sun-
light, and no indication of light saturation appears to be reached (Murata, 1961). In early stages of
crop growth, the main determinant of photosynthesis is the extent of leaf area development. As the
LAIs increase, so do the extents of light interception, and often exceed 95% for many cereal crops
with LAI values of about 4. Once canopies close from leaf density, further increases in LAI have
little effect on plant photosynthesis, which may be influenced by incident radiation and structures of
canopies (Evans and Wardlaw, 1976).
Net photosynthesis rates of active, healthy single rice (C
3
plant) leaves are about 40-50 mg CO
2
dm
−2
h
−1
under light saturation conditions. Ishii (1988) noted no significant differences in photo-
synthetic rates of single rice leaves at heading stage of growth for 32 Japanese cultivars bred from
1880 to 1976. However, 25% increases in canopy photosynthesis were observed in new rice cultivars
(bred between 1949 and 1976) as compared to old cultivars (bred from 1880 to 1913). Improvement
in canopy photosynthesis was attributed to the higher efficiency of light interception by canopies
because of changes in leaf angles. Old cultivars had droopy leaves with high mutual shading, while
new cultivars had erect leaves with less mutual shading (Ishii, 1988). Similarly, Conocono et al.
(1998) concluded that much of the large increases in rice yield over the past three decades could
be attributed to improvements in canopy structure that enhanced canopy light interception and
photosynthesis.
Photosynthesis provides 90-95% of plant dry weight (Kueneman et al., 1979). N is arguably
essential to the entire photosynthetic apparatus, since it is contained in individual pyrrole subunits
that form the tetrapyrrole ring structure common to all light-absorbing chlorophyll molecules (Rice,
2007). The tetrapyrrole ring gives chlorophyll its green color, and the term chloritic suggests an
absence of green, a typical manifestation of N deficiency that reflects compromised chlorophyll
function within chloroplasts (Rice, 2007).
Many studies have related the increase in CO
2
assimilation rate of many crops to increase
in leaf N (Osman and Milthorpe, 1971; Rawson and Hackett, 1974; Yoshida and Coronel, 1976;
Evans, 1983). Whenever a sufficiently broad range of leaf N contents has been examined, it has
